Fix red develop 171d416138ee

The merge lane found `develop` red at `171d416138ee65b57cd3e1d6273b9d7ca7be7518` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/test_cross_org_isolation.py::test_no_route_lets_a_person_leave_an_org_themselves` - `api-unit-dc-test` The whole gate log: artifact `7659` on lane task 8554. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Fixed red develop: the leave-door census now admits DELETE /api/auth/signup/pending (clears a signup cookie only). Targeted test + quick gate green; PR #6480 queued to the merge lane. Memory: none applied.